The move means the Chinese aluminium giant has broken ranks with a body that represented companies producing more than 60% of the metal and its raw materials bauxite and alumina worldwide, prior to Hongqiao's departure.

Hongqiao, which has about 6.5 million tonnes of licensed annual aluminium smelting capacity, became an IAI member in 2016 and now employs former IAI secretary-general Ron Knapp as an adviser to the company's chairman, Zhang Bo.

Knapp confirmed to Reuters that Hongqiao had resigned from the IAI but declined to comment on the reasons.

"What Hongqiao is about is increasing its engagement internationally and we are looking at a number of vehicles to assist in realising that goal," he said.

The IAI, whose website no longer lists Hongqiao as a member, declined to comment on Tuesday.

Its objectives include increasing the market for aluminium by enhancing awareness of its qualities and providing a global forum for aluminium producers on matters of common concern for its now 26 members.

China is by far the world's biggest aluminium producing and consuming country.

Hongqiao chairman Zhang previously served as vice chairman of the IAI, which is currently chaired by Alcoa Corp executive Ben Kahrs and has a secretariat in London.

Other members include Aluminum Corp of China, known as Chinalco, Rio Tinto, United Company Rusal and Norsk Hydro.

Other organisations Shandong-based Hongqiao would like to work with include the Aluminium Stewardship Initiative, a standards-setting body that aims to foster responsible production and sourcing of the metal, Knapp said.

(Reporting by Tom Daly; Additional reporting by Pratima Desai in London; Editing by David Clarke)